Automotive6 AluReport 03.2012

7xxx-high strength aluminum sheets for lightweight automotive applications

Currently, for sheet applications in car

body engineering AlMg alloys of the

5xxx series or precipitation harden-

ing AlMgSi alloys of the 6xxx series

are used. The focus on lightweight design is

still increasing due to legislation and customer

requirements for less fuel consumption. There-

fore high strength AlZnMg(Cu) alloys of the

7xxx series with tensile strength up to 700

MPa are extensively discussed. The aerospace

and sports industry have been benefitting from

the utilization of these high strength alloys for

decades in terms of significant weight savings

and enhanced performance.

However a successful transfer to the automo-

tive industry requires innovative solutions to

allow cost-efficient series production. The pro-

posed solution is such that rolling, solution heat

treatment and artificial ageing shown in figure

1 are done at the rolling mill, while the com-

ponents are produced via warmforming at the

customer‘s site. Warm forming at temperature

levels far below those for press hardening steel

helps to overcome the moderate formability of

7xxx-alloys at RT.

High strength 7xxx series alloys of the type

AlZnMg(Cu) double the yield strength com-

pared to standard 6xxx series automotive al-

loys. Both alloy families increase their strength

significantly by precipitation hardening. In a

continuous strip annealing line both alloy types

are solution heat treated and quenched to

freeze the supersaturated solid solution. Natu-

ral ageing at room temperature starts and in

contrast to AlMgSi alloys formable for a couple

of months the hardening of AlZnMg(Cu) al-

loys continues. Therefore, AMAG developed

a Cu-containing alloy called AMAG TopForm®

UHS for replacing press hardened steel in

automotive components like B-pillars or side

impact beams. It is an AA7075 type alloy

(AlZn5,5MgCu) optimized for warm forming

in the long-term stable T6 peak age delivery

temper. Higher strength compared to Cu-free

derivatives combined with controlled solution

annealing, quenching and artificial ageing at

aircraft certified continuous coil treating lines

ensure reproducible constant properties and

reduce the investments and processes needed

at the car manufacturer.

Automotive 7AluReport 03.2012

Warm Forming and Paint Bake ResponseCold forming of artificially aged AA7075 in tem-

per T6 is limited to rather simple geometries,

e.g. roll forming with radii according to string-

ers in the aircraft industry. For complex compo-

nents warm forming is recommended.

Based on FEM simulation results the param-

eters for warm forming at a pilot press line were

adjusted. The plane 7xxx-blank was heated up

to the warm forming temperature within a min-

ute in a simple hydraulic press in direct contact

to hot plates. Short process times and a low

process temperature of around 200°C are es-

sential to keep the decrease of strength due to

overaging as small as possible.

Figure 1: Temperature-time-diagram for high strength 7xxx series alloys

400°C

200°C

20°C

Rolling Mill

minutes

Solution

Annealing

minuteshours

Costumer

Warm

Forming

Water

Quench

Artificial Ageing

Automotive8 AluReport 03.2012

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ajo

r str

ain

1

minor strain 2

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � �! " # # $ $ " % & ' ( # ! ! ) * *Figure 2: Forming limit diagram for AMAG TopForm® UHS at different temperatures vs. AA6016 T4

Figure 3: Side impact protection made from AMAG TopForm® UHS

Figure 4: Warm forming plus 5-step paint bake cycle for AMAG TopForm® UHS

+, + +- + +. + +/ + +0 + +1 + + 2 3 4 5 6 7 8 2 9 : ; < 4 5 6 7 8= >?= @A BCDEF GH

The forming limit curves (FLC) in Figure 2

compare the formability of AMAG TopForm®

UHS at different temperatures around 200°C

with the standard cold forming procedure of a

typical AA6016 automotive alloy in temper T4.

As already mentioned, cold forming of an

AA7075 T6 sheet is limited to rather simple

geometries. At 170°C the forming limit curve

comes close to the AA6016 T4 curve es-

pecially for plain strain conditions. A further

increase to 230°C improves the formability

significantly and even for the stretch-forming

path AMAG TopForm® UHS reaches the

formability of AA6016 T4 in cold forming.

On a pilot warm forming line a small series of a

structural component, similar to a side impact

beam of an automotive door was produced at

a Tier 1 automotive supplier in co-operation

with AMAG (Figure 3). This process proved to

be robust with very small variation of mechani-

cal properties after forming and painting.

While a variation of the warm forming temper-

ature leads to different mechanical properties

after the press shop, after the additional heat

treatment in the paint shop the final material

properties are very uniform. Figure 4 com-

pares the tensile and yield strength after warm

forming and after an additional heat treatment

in the paint shop.

While warm forming at 170°C with just one

or two minutes process time has almost no

impact on strength, a slight increase to 200°C

reduces the strength by some 50 MPa. Typi-

cally a common 5-step paint-bake cycle for

drying the body-in-white structure, e-coating

and curing the various paint layers enables

re-aging. About altogether one hour heating

with a temperature collective of 125 to 185°C

with intermediate cooling to room temperature

leads to a yield-strength of 460 MPa.

Automotive 9AluReport 03.2012

From the customers point of view the process

starts with well defined and long-term stable

properties of AMAG TopForm® UHS sheet.

During warm forming in the customer‘s press

shop and the susequent paint bake cycle the

mechanical properties of the component be-

come uniform on a high level.

Parameter fluctuations of time and tempera-

ture during warm forming do not result in fluc-

tuations of the material properties after form-

ing and painting (Figure 4).

It was also observed that the springback after

warm forming is low because of the signifi-

cantly reduced yield strength at forming tem-

perture.

JoiningConventional fusion welding of copper con-

taining 7xxx series is difficult due to the oc-

currence of solidification cracks. For AMAG

TopForm® UHS two modified thermal joining

methods were tested on a laboratory scale.

Resistance spot welding trials with the al-

ready commercially available Fronius Deltas-

pot® technology show very promising results.

Thereby two cross die parts with a drawing

depth of approx. 50 mm, derived from a 2 mm

AMAG TopForm® UHS sheet, were produced

and joined (Figure 5). Detailed results will be

presented in the next AluReport 1/2013.

Additionally, AMAG TopForm® UHS was suc-

cessfully joined with Friction Stir Spot Welding

(FSSW). This is a modification of the well-

known FSW process resulting in round join-

ing spots. A process innovation of the com-

pany RIFTEC with a segmented rotating tool

fills the deepening of the spots leading to a

smooth surface. This improved joining method

shows promising results for overlap joints of

AA7075 sheets.

The aircraft industry has a positive long-term

experience with mechanical joining methods

(e.g. riveting) of 7xxx series Al-alloys which

are also applicable to AMAG TopForm® UHS.

Automotive specialties like self piercing rivets

or flow drill screws should also work but have

to be adapted from low and medium strength

aluminium to high strength 7xxx series alloys.

Adhesive bonding is another widely spread

joining technology used for aluminium in the

aerospace industry. In recent years this type of

fact that there is no requirement of rapid

quenching after warm forming.

A reliable and stable heat treatment process

at the rolling mill provides long-term stable

mechanical delivery properties in T6 temper

and offers stable high level properties after

press and paint shop within a robust pro-

cess window. New innovative thermal join-

ing technologies recently introduced to the

automotive industry have been successfully

tested for this AMAG high strength alloy.

Moreover, mechanical methods and hybrid

joining in conjunction with adhesive bond-

ing extend the joining portfolio. AMAG Top-

Form® UHS in combination with tailored

pre-treatments fully exploits the strength

potential of modern automotive adhesives.

AMAG TopForm® UHS doubles the strength

compared to a standard AA6016 alloy.

Therefore a high specific resistance to dent-

ing and high specific crash performance

make this alloy ideal for the replacement

of press hardened steels e.g. in side im-

pact protections or bumper beams. AMAG

TopForm® UHS is an AA7075 type al-

loy (AlZn5,5MgCu) optimized for excellent

warm forming behaviour at temperatures

between 170 and 230°C.

While time consuming process steps at high

temperatures like solution annealing and ar-

tificial ageing are done in the rolling mill in an

efficient and controlled manner it just takes

seconds in the press shop to heat up the

blank to a temperature of around 200°C.

Furthermore the customer benefits from the

Figure 5: Resistance spot welded (Fronius Deltaspot®) AMAG TopForm® UHS cross die

joining became popular in car manufacturing

especially in multimaterial car body designs.

Galvanic isolation of materials with different

electrochemical potentials and the prevention

of crevice corrosion is an important factor for

this type of joining. For a sufficient degrada-

tion performance of the bond, the surface has

to be properly prepared.

Adhesive bonding pre-treatment in the air-

craft industry is based on batch wise anodiz-

ing procedures for structural parts exhibiting

excellent tensile shear strength and fracture

pattern. So far, new pre-treatments show

comparable good results on a laboratory scale

and tests on AMAG’s modern and flexible

continuous automotive pre-treatment line will

follow soon.